Lecture 2: Links and Signaling. CSE 123: Computer Networks Stefan Savage

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1 Lecture 2: Links and Signaling CSE 123: Computer Networks Stefan Savage

2 Lecture 2 Overview Signaling Channel characteristics Types of physical media Modulation Narrowband vs. Broadband Encoding schemes A lot of this material is not in the book Caveat: I am not an EE Professor CSE 123 Lecture 1: Course Introduction 2

Today s Goal: Send bits A three-step process Take an input stream of bits (digital data) Modulate some physical media to send data (analog) Demodulate the signal to retrieve bits (digital again)

3 Today s Goal: Send bits A three-step process Take an input stream of bits (digital data) Modulate some physical media to send data (analog) Demodulate the signal to retrieve bits (digital again) Anybody heard of a modem (Modulator-demodulator)? digital data (a string of symbols) modulation a signal demodulation digital data (a string of symbols) CSE 123 Lecture 2: Links and Signaling 3

4 A Simple Signaling System CSE 123 Lecture 2: Links and Signaling 4

5 Another Simple Signaling System CSE 123 Lecture 2: Links and Signaling 5

6 Another Simple Signaling System CSE 123 Lecture 2: Links and Signaling 6

7 Binary signaling with Voltage Encode 1 s and 0 s on a wire +5 volts = 1-5 volts = 0 CSE 123 Lecture 2: Links and Signaling 7

8 Signals and Channels A signal is some form of energy (light, voltage, etc) Varies with time (on/off, high/low, etc.) Can be continuous or discrete A channel is a physical medium that conveys energy Any real channel will distort the input signal as it does so How it distorts the signal depends on the signal CSE 123 Lecture 2: Links and Signaling 8

9 Channel Challenges Every channel degrades a signal Distortion impacts how the receiver will interpret signal response ideal actual B freq CSE 123 Lecture 2: Links and Signaling 9

10 Channel Properties Bandwidth-limited Range of frequencies the channel will transmit Means the channel is slow to react to change in signal Power attenuates over distance Signal gets softer (harder to hear ) the further it travels Different frequencies have different response (distortion) Background noise or interference May add or subtract from original signal Different physical characteristics Point-to-point vs. shared media Very different price points to deploy CSE 123 Lecture 2: Links and Signaling 10

pair coaxial cable (coax) copper core insulation braided

11 Copper Typical examples Category 5/6 Twisted Pair 10M-10Gbps m Coaxial Cable Mbps 200m twisted pair coaxial cable (coax) copper core insulation braided outer conductor outer insulation CSE 123 Lecture 2: Links and Signaling 11

(LED vs laser) & terminate Longer distance (low attenuation)

12 Fiber Optics Typical examples Multimode Fiber 100Mbps-10Gb m Single Mode Fiber 1-100Gbps 100m-40km Cheaper to drive (LED vs laser) & terminate Longer distance (low attenuation) Higher data rates (low dispersion) CSE 123 Lecture 2: Links and Signaling 12

13 Wireless Widely varying channel bandwidths/distances Extremely vulnerable to noise and interference AM FM Twisted Pair Coax TV Microwave Satellite Fiber Freq (Hz) Radio Microwave IR Light UV CSE 123 Lecture 2: Links and Signaling 13

land, disk space, etc) n Reality is that spectrum is time and power shared n

14 Spectrum Allocation n Policy approach forces spectrum to be allocated like a fixed spatial resource (e.g. land, disk space, etc) n Reality is that spectrum is time and power shared n Measurements show that fixed allocations are poorly utilized0 Frequency (Hz) Hot topic: Whitespace communication Time (min) CSE 123 Lecture 2: Links and Signaling 14

15 Two Main Tasks First we need to transmit a signal Determine how to send the data, and how quickly Then we need to receive a (degraded) signal Figure out when someone is sending us bits Determine which bits they are sending A lot like a conversation WhatintheworldamIsaying needs punctuation and pacing Helps to know what language I m speaking CSE 123 Lecture 2: Links and Signaling 15

16 The Magic of Sine Waves All periodic signals can be expressed as sine waves Component waves are of different frequencies Sine waves are nice Phase shifted or scaled by most channels Easy to analyze Fourier analysis can tell us how signal changes But not in this class CSE 123 Lecture 2: Links and Signaling 16

Carrier Signals Baseband modulation: send the bare signal E.g. +5 Volts for 1, -5 Volts for 0 All signals fall in the same frequency range Broadband modulation Use the signal to modulate a high frequency signal (carrier).

17 Carrier Signals Baseband modulation: send the bare signal E.g. +5 Volts for 1, -5 Volts for 0 All signals fall in the same frequency range Broadband modulation Use the signal to modulate a high frequency signal (carrier). Can be viewed as the product of the two signals Amplitude Amplitude Signal Carrier Frequency Modulated Carrier CSE 123 Lecture 2: Links and Signaling 17

18 Forms of Digital Modulation Input Signal Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) CSE 120 Lecture 1: Course Introduction 18

19 Why Different Schemes? Properties of channel and desired application AM vs FM for analog radio Efficiency Some modulations can encode many bits for each symbol (subject to Shannon limit more on this next class) Aiding with error detection Dependency between symbols can tell if a symbol wasn t decoded correctly Transmitter/receiver Complexity CSE 123 Lecture 2: Links and Signaling 19

20 For Next Class Read 2.3 Log into Piazza; let us know if you have problems CSE 123 Lecture 2: Links and Signaling 20

Lecture 2: Links and Signaling"

Lecture 2: Links and Signaling" CSE 123: Computer Networks Alex C. Snoeren HW 1 out tomorrow, due next 10/9! Lecture 2 Overview" Signaling Types of physical media Shannon s Law and Nyquist Limit Encoding